Thanks everyone. I got about 20 MMS and SMS messages to figure out how to support these types of emails for dasBlog. I actually found a few minor bugs in the Mail-To-Weblog support and while the messages of course don't come out as pretty as from a regular email client, it does at least work now.

What's key -- and that's something that I can't figure out a consistent workaround for -- is that the SMS/MMS gateway must be capable of setting a subject line for emails so that dasBlog can do it's passphrase check. For Germany, this page has a bunch of info on how that works in the major networks (in German).